Drug delivery devices allowing for multiple dosing of a required dosage of a liquid medicinal product, such as a liquid medicament, and further providing administration of the liquid to a patient, are as such well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Drug delivery devices of this kind have to meet a number of user specific requirements. For instance in case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Further, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
Typically, the medicinal product to be administered is provided in a cartridge that has a moveable piston or bung mechanically interacting with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the piston in distal direction, a certain amount of the medicinal fluid is expelled from the cartridge and may be administered to the patient by some kind of needle assembly being in fluid communication with the cartridge.
Due to inevitable manufacturing tolerances of the device and the cartridge there may for instance persist axial clearance between a cartridge's piston and the piston rod when the device is finally assembled. Typically, prior to a primary use of the device, an end-user, e.g. a patient has to conduct a so-called set-up of the drive mechanism in order to ensure, that the piston of the cartridge and the piston rod are located at a pre-defined position with respect to each other, thus ensuring, that with an initial dose setting and a subsequent dose dispensing step, a predefined amount of the medicinal product can be disposed in an accurate way. By way of the set-up step, mechanical tolerances of movable components of the drug delivery device can be annihilated.
In particular with disposable drug delivery devices, wherein the entire device is discarded after consumption of the medicament, the device is initially provided with a cartridge containing the medicament. Such disposable but also reusable drug delivery devices are assembled in a mass production process in which for instance two housing components of the drug delivery device receive a cartridge and a drive mechanism including the piston rod, respectively. Then, in a final step of assembly, the two pre-configured housing components or respective sub-assemblies are to be mutually interconnected. When reaching a defined final assembly configuration, it would be beneficial when the piston rod and the piston of the cartridge either mutually abut or when piston and piston rod are separated by a pre-defined gap.
Exact and precise mutual positioning and alignment of piston and piston rod is important and crucial for accurate and reliable functionality of the drug delivery device. Moreover, the piston rod should not exert pressure to the plunger during assembly, which may otherwise result in a rather uncontrolled expelling of the medicament prior to a first use of the drug delivery device when a needle is attached.
Since at least some or even major components of the drug delivery device are designed as plastic injection molded components, the components themselves, and their assembly is inevitably subject to certain geometric tolerances. Moreover, the cartridge itself and in particular the position of the piston within the cartridge may vary.
It is therefore an object of the present invention to provide an improved piston rod assembly for a drug delivery device which may allow compensating for geometric tolerances of the drug delivery device and its components. In a further object, the piston rod assembly and the respective drug delivery device should be intuitive and easy in handling. Furthermore, a general compatibility of the piston rod assembly and of a respective drug delivery device with existing manufacturing processes is a further aim of the invention. Finally, the invention should be implementable with reasonable costs and expenditure.